The present invention relates generally to aqueous metal cleaning compositions. In particular, this invention is directed to aqueous metal cleaning compositions useful in so-called parts washers which are particularly adapted to be used for industrial cleaning, as well as for domestic use.
Parts washers of various kinds are known to those skilled in the art as having great utility for mechanics and others working in a variety of occupations, particularly those working in industrial plants, maintenance and repair services, and the like. The parts washers referred to herein include soak tanks, so-called hot tanks, immersion type parts cleaners with or without air agitation, spray washers (continuous or batch) and ultrasonic baths. Generally, parts washers are used to remove all types of contaminants adhered to the metal surface including greases, cutting fluids, drawing fluids, machine oils, antirust oils such as cosmoline, carbonaceous soils, sebaceous soils, particulate matter, waxes, paraffins, used motor oil, fuels, etc.
Until recently, metal surfaces were cleaned of most oily and greasy contamination by use of solvents. Existing solvents, with or without special additives, are adequate to achieve good cleaning of most dirty, greasy, metal parts. A great number of solvents have been employed to produce metallic surfaces free from contamination. These wash solvents generally include various halogenated hydrocarbons and non-halogenated hydrocarbons, of significant quantity industry wide for cleaning and degreasing of the metal surfaces, and the degree of success with each of these wash solvents is generally dependent upon the degree of cleanliness required of the resultant surface.
Recently, however, the various hydrocarbon and halogenated hydrocarbon metal cleaning solvents previously employed have come under scrutiny in view of the materials employed, and in particular, the environmental impact from the usage of the various materials. This is particularly so in the case of parts cleaning which is done in closed environments such as garages and the like or for even home usage in view of the close human contact. Even the addition of devices to parts washers which can, reduce spillage, fire and excessive volatilization of the cleaning solvent are not sufficient to alleviate present environmental concerns.
Although the halogenated hydrocarbon solvents such as chlorofluorocarbons (CFCs) and trichloromethane, methylene chloride and trichloroethane (methyl chloroform) are widely used in industry for metal cleaning, their safety, environmental and cost factors coupled with waste disposal problems are negative aspects in their usage. A world-wide and U.S. ban on most halogenated hydrocarbon solvents is soon in the offing by virtue of the Montreal Protocol, Clean Air Act and Executive and Departmental directives.
The non-halogenated hydrocarbon solvents such as toluene and Stoddard solvent and like organic compounds such as ketones and alcohols on the other hand are generally flammable, have high volatility and dubious ability to be recycled for continuous use. These, plus unfavorable safety, environmental and cost factors, put this group of solvents in a category which is unattractive for practical consideration. Most useful organic solvents are classified as volatile organic compounds (VOCs) which pollute the atmosphere, promote formation of toxic ozone at ground level, and add to the inventory of greenhouse gases.
In order to eliminate the various negative aspects of the known chemical washing and degreasing systems, it has, therefore, been suggested that an aqueous detergent system be used so as to overcome some of the inherent negative environmental and health aspects of prior art solvent cleaning systems. Unfortunately, aqueous cleaning systems are not without their own problems as related to use thereof in metal cleaning systems including use in parts washers as described above. For example, certain of the aqueous cleaners are exceedingly alkaline having pHs of 13 and above such as sodium hydroxide or include organic solvents such as alkanolamine, ethers, alcohols, glycols, ketones and the like. Besides being highly corrosive, the exceedingly high alkaline aqueous solutions are highly toxic and can be dangerous to handle requiring extreme safety measures to avoid contact with skin. Organic solvent-containing aqueous cleaners present the problems regarding toxicity, volatility or the environment as expressed previously. On the other hand, it is most difficult to obtain an aqueous detersive solution at moderate pH which is effective in removing the greases and oils which contaminate metal including metal engine parts and which would not be corrosive to the metal substrate.
One particular disadvantage of using aqueous systems to clean metal surfaces is the potential to corrode or discolor the surfaces. While aqueous cleaning solutions having a high pH such as formed from sodium hydroxide are often more corrosive than aqueous solutions having a moderate pH such as formed by mildly alkaline detergents, corrosion and discoloration are still problematic with the more moderate solutions.
Various corrosion inhibitors are known and have been used to prevent corrosion of metal surfaces which come into contact with aqueous alkaline solutions. This is because no one inhibitor, or combination of inhibitors, yet has provided protection for all metals and metal alloys. Examples of corrosion inhibitors include inorganic compounds such as alkali metal phosphates, borates, molybdates, arsenates, arsenites, nitrates, silicates, nitrites, and chromates, as well as various organic compounds such as mercaptobenzothiazole, benzotriazole, piperazine, ethylene diamine tetraacetic acid and the reaction product of phosphoric acid or boric acid and an alkanolamine.
Accordingly, to be as effective and be able to replace the halogenated and hydrocarbon solvents now widely used, aqueous metal cleaning compositions will have to be formulated to solve the problems associated therewith including efficacy of detersive action at moderate pH levels and the corrosiveness inherent in aqueous based systems, in particular, on metal substrates.
One particular problem with respect to corrosion using aqueous metal cleaning solutions is manifest in the cleaning of iron-based metals. Thus, it has been found that iron-based metals treated with aqueous based systems and then removed from the aqueous solution begin to rust almost immediately. This phenomenon has been characterized as flash rusting. Inasmuch as it takes longer for metal parts to dry subsequent to treatment with aqueous based cleaners as compared to the drying times of organic solvent-based cleaners due to the high surface tension of water, the potential for flash rusting to occur with iron-containing metal substrates is a serious drawback to the use of aqueous based cleaners to clean such metal surfaces.
It is also important that the aqueous metal cleaners be reusable to render such cleaners economically viable. Thus, it is not practical on an industrial scale to sewer an aqueous cleaning bath upon a single usage thereof. Many of the aqueous based cleaners now available use detersive agents which are effective in removing the dirt, grease or oil from the metal surface but unfortunately the contaminants are highly dispersed or solubilized throughout the aqueous solution. Such cleaning solutions are difficult to treat to separate contaminants from the aqueous cleaner and, accordingly, the cleaning solution gets spent in a relatively short period of time and must be replaced to again achieve effective cleaning of the metal parts and the like. It would be worthwhile to provide an aqueous metal cleaner which could effectively remove the contaminants from the metal surface and allow formation of a separate distinct and substantially complete contaminant phase from the cleaning solution phase to permit effective and prolonged reuse of the cleaning solution.
Still another disadvantage of the use of aqueous cleaners again stems from the high surface tension of water and the propensity of the detersive agents in the aqueous cleaner to foam upon agitation of the cleaning bath such as induced in the bath or by the use of spray nozzles to apply the cleaning solution to the metal components being cleaned. The presence of foam often renders the use of machines with high mechanical agitation impractical due to excessive foaming. Also, the presence of foam can cause pump cavitation problems and the overflow of liquids onto floors as well as cause difficulties with viewing the cleaning process through vision ports and the like contained in the machinery.
Accordingly, it is an object of this invention to provide an aqueous metal cleaning composition which is effective to clean grease, oil, dirt or any other contaminant from a metal surface and yet have a relatively moderate pH so as to not be excessively corrosive to the substrate and irritating to human skin.
Another object of the invention is to provide an aqueous metal cleaning composition which can be used effectively in immersion and impingement type parts washers so as to effectively remove dirt, grease, oil and other contaminants from metal parts and which is safe to use and not a hazard to the environment in use or upon disposal.
Still another object of the present invention is to provide an aqueous metal cleaning composition which is not corrosive to metal parts in general and, in particular, can greatly reduce flash rusting of iron-containing metal components.
Still yet another object of the present invention is to provide an aqueous metal cleaning composition of moderate pH which has effective detersive action and is low foaming to maintain the cleaning efficacy of the composition in aqueous solution.
A further object of the present invention is to provide an aqueous metal cleaning composition where contaminants removed from a metal surface form a phase separate from the aqueous phase containing the cleaning composition such that the contaminants can be separated from the aqueous cleaning solution and the solution continuously reused.
Yet another object of this invention is to provide an aqueous cleaning concentrate which when diluted to cleaning concentration can be an effective and environmentally sound aqueous cleaner.
These and other objects of the present invention can be readily ascertained from the description of the invention which follows.